1. Technical Field
The present invention relates to a liquid developer and an image forming apparatus, and in particular relates to a liquid developer and an image forming apparatus that can use the liquid developer.
2. Related Art
As a developer used for developing an electrostatic latent image formed on a latent image carrier, there are known two types. One type of such a developer is known as a dry toner which is formed of a material containing a coloring agent such as a pigment or the like and a binder resin, and such a dry toner is used in a dry condition thereof.
The other type of such a developer is known as a liquid developer (liquid toner) which is obtained by dispersing toner particles into a carrier liquid having electric insulation properties (one example of such a liquid developer is disclosed in JP-A 2006-251253).
In the developing method using such a dry toner, since a solid state toner is used, there is an advantage in handleability thereof. On the other hand, however, this method involves problems in that an adverse effect against a human body is likely to be caused by toner powder, contamination is likely to occur by dispersal of toner powder, and toner particles are likely to be massed together in a cartridge.
Further, in such a dry toner, since aggregation of toner particles is likely to occur in the producing process thereof, it is difficult to obtain toner particles each having a sufficiently small diameter. This means that it is difficult to form a toner image having high resolution.
Furthermore, there is also a problem in that when the size of the toner particle is made to be relatively small, the problems resulted from the powder form of the dry toner described above become more serious.
On the other hand, in the developing method using the liquid developer, since aggregation of toner particles in the liquid developer is effectively prevented, it is possible to use very fine toner particles and it is also possible to use a binder resin having a lower softening point (a low softening temperature).
As a result, the method using the liquid developer has such advantages as good reproducibility of an image composed of thin lines, good tone reproducibility as well as good reproducibility of colors. Further, the method using the liquid developer is also superior as a method for forming an image at high speed.
However, in the conventional liquid developer an insulation liquid contained in the liquid developer has low affinity to the toner particles, and thus it is difficult for the liquid developer to maintain the good dispersing state of the toner particles in the liquid developer for a long period of time. As a result, it is difficult to sufficiently maintain preservability or storage stability of the liquid developer for a long period of time.
Additionally, in view of the energy saving trend of recent years, liquid developers are required to have fixing characteristics at a relatively low temperature. However, in the currently available liquid developers, there is a problem in that offsets (low temperature offsets) are likely to occur in fixing at a low temperature.